“Game of Thrones” went cosmic this season.Sure, 18 million viewers (18.4 million to be exact) is a lot of viewers — the most of any HBO series in history, surpassing “The Sopranos” just this week. But you know on a personal level that something has weirdly changed when you hear a guy on the subway next to you talk about the introductory course in High Valyrian Dothraki he’s taking online.Why do shows suddenly alter the world we inhabit? This indisputably happened during the fourth season of “Game of Thrones,” which wraps Sunday. Today, a quick primer on why “GoT” has become aeksio — Dothraki for lord and master — of pop culture.It zigged, then zagged, from the book: The fourth season has largely followed events depicted in “A Storm of Swords,” the third volume in George R.R. Martin’s “A Song of Ice and Fire,” upon which “GoT” is based — but there have been many minor departures, too. Yes, this has happened every season, but this season: much more. From a viewer/fan standpoint, that’s good in a couple of ways. Obviously, plot and narration can be condensed in the editing room to make the whole more TV-friendly (which happens each season), but showrunners D.B. Weiss and David Benioff have added new elements, stories and scenes this year. These tend to keep fans familiar with the books guessing, but also serve to reinforce the alluring idea that “GoT” inhabits a slightly separate universe from “Ice and Fire.” The implicit message, therefore, becomes: Don’t expect the exact outcome you all think is coming.The fourth season is based on a hugely popular book: “A Storm of Swords,” published in 2000, remains a fan favorite for many reasons — notably it is probably one of the most cinematic books of the series, which I suppose is just another way of saying that a lot of wild and interesting stuff happens. (The volume that follows, “A Feast for Crows,” is the least favorite, so Weiss and Benioff have an interesting challenge next season.) Because “Swords” is so popular — and it is as fun to read as to watch — then it stands to reason that the resulting season should be as well.Familiarity: The tipping point has been reached. Viewers no longer have to scramble over to “A Wiki of Ice and Fire” a find out what a “House Baratheon of Storm’s End” is or who “Cersei” or “Sansa” is. They know the characters (and many of the Houses) intimately — their story lines especially. There’s nothing even vaguely remote about Westeros any longer. It’s the epic fanatasyland next door.The fourth season just kept building ... And building: Narratively, the fourth season has been — bluntly speaking — nuts. Almost every episode has begun with a jaw-dropper, ended with one and dropped a few more jaws in between. Last Sunday’s “The Watcher’s on the Wall” was TV’s best roller coaster ride in years (it featured the death of wildling Ygritte, played by Rose Leslie). The fourth has simply been relentless, and while Sunday’s finale would normally be the moment to pause, reflect, take a deep breath and take stock — the usual practice for “GoT” — viewers will not be afforded that luxury. In a statement (posted on EW.com Wednesday,) Benioff and Weiss said, “We’re immensely proud of (the finale, entitled ‘The Children’) and a little intimidated by the episode, because now we have to get back to the business of season five and figure out a way to top it.”It was deeper and richer: Great series, like good books, are about more than what’s on the screen or on the page — a philosophy, a worldview and a sense that all this is really just an exploration of something more elemental, like the human condition. Martin has long said his yarn is a distant echo of the War of the Roses, the dynastic battles between the houses of Lancaster and York from the mid-1400s. Shakespeare also read deeply into those, of course, basing any number of historical plays on that long and bloody struggle; Richard III, for example, who has been compared widely to Tyrion Lannister (Peter Dinklage), and even more to Stannis Baratheon (Stephen Dillane). “GoT” openly flirted with Shakespearean themes this season (most famously Tyrion’s “beetles” speech two weeks ago). The flirtation has not only been exhilarating, but opened doors that thinking viewers can’t wait to walk through.

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